This invention relates to radiological well logging methods and apparatus for investigating the characteristics of subsurface earth formations traversed by a borehole and more particularly relates to improved pulsed neutron well logging techniques for differentiating and determining the quality of coal and oil shale regions in subsurface earth formations traversed by a well borehole.
It has been proposed in the prior art to make a measurement of at least a portion of the gamma ray energy spectrum due to inelastic neutron scattering events from neutron irradiated earth formations. This has been proposed because carbon and oxygen have significant inelastic scattering cross-sections while having a relatively small capture cross-section for high energy neutrons. Thus, the carbon and oxygen nuclei in formations surrounding a borehole will engage in appreciable inelastic scattering interactions with the bombarding high energy neutrons. Gamma rays resulting from the inelastic neutron scattering interaction (henceforth referred to as inelastic gamma rays) may then be detected as a direct hydrocarbon indicator. However, this approach has been limited in the past to some extent because the inelastic scattering cross-section of carbon and oxygen only became appreciable if relatively high energy neutrons are available to provide the inelastic scattering interaction. In the past it has been difficult to provide sufficient quantities of energetic neutrons to reliably perform this type of well log. The development of improved pulsed neutron generators has made possible the measurement of the inelastic scattering gamma ray energy spectrum from relatively high energy neutron irradiated earth formations. It is now possible to measure the carbon and oxygen inelastic scattering interactions with 14 MEV neutrons generated in pulsed neutron generators of the deuterium-tritium reaction type.
A difficulty encountered in this type of direct hydrocarbon logging has been due to the fact that carbon is present in significant amounts in the earth's crust in other than hydrocarbon bearing formations. For example, limestone formations are largely composed of calcium carbonate and thus a water bearing limestone formation can produce more inelastic carbon gamma rays than an oil filled shale or sand. The carbon/oxygen ratio of inelastic gamma rays has also been found to be a function of porosity, and therefore a low porosity oil filled formation may produce less inelastic carbon gamma rays than a partially depleted higher porosity formation.
Another problem in making inelastic gamma ray measurements has been due to the fact that the gamma rays generated by the neutron inelastic scattering can suffer multiple Compton scattering. Such scattering generally tends to make the scattered gamma rays lose energy to some extent with each interaction. Thus, a gamma ray having an initial particular energy generated by the inelastic scattering of a neutron by a carbon or oxygen nucleus can have an apparently different energy by this time it reaches a detector in a well logging sonde. However, techniques such as those disclosed, for example, in U.S. Pat. No. 3,780,303 have been derived to generally alleviate the aforementioned problems. Using techniques of the type described in this aforementioned patent, it is possible to derive a hydrocarbon log of subsurface earth formations in the vicinity of a well borehole by observing the inelastic gamma rays due to the scattering of 14 MEV neutrons from a pulsed neutron source employing the deuterium-tritium reaction.
In the present invention the carbon/oxygen ratio of subsurface earth formations can be used to determine the quality or BTU content of any coal beds encountered by a well borehole in penetrating earth formations and also to determine the quality or gallons per ton of producable oil in an oil shale formation which is encountered by the well borehole. Means are provided for distinguishing shale and washed out regions of formations in the vicinity of a borehole and for distinguishing limestone and other high carbon content earth formation such as dolomite which may be associated with oil bearing shales or sands. By combining these measurements with a porosity or hydrogen index log the moisture content of the coal or oil shale zones may be estimated.
Accordingly, it is an object of the present invention to provide methods for determining the quality of coal bearing earth formations in the vicinity of a well borehole.
Another object of the present invention is to provide an improved technique for determining the quality of oil shale formations encountered by a well borehole.
The above and other objects of the present invention are accomplished by passing a well tool having a pulsed source of 14 MEV neutrons through a fluid filled cased or uncased well bore. Inelastic gamma ray measurements of neutrons produced from the pulsed neutron generator are observed in at least four different energy regions in the gamma ray energy spectrum corresponding to inelastic scattering gamma rays produced by carbon, oxygen, silicon and calcium. The carbon/oxygen count ratio of these inelestic gamma rays is formed. The calcium/silicon count ratio of the inelastic gamma rays is formed and the sum of the carbon plus oxygen inelastic gamma rays is formed. The above quantities may be recorded as a function of depth in the well borehole. Separately derived well logs indicative of the hydrogen index of the formations of interest may then be used in conjunction with the inelastic gamma ray scattering measurements in order to distinguish coal bearing and oil shale formations in the vicinity of the well borehole from other earth formations and to derive a quantitative estimate of the quality of the BTU content of the coal bearing formation and the amount of oil present in the oil shale formations.
The present invention may be best understood by taking the following detailed description thereof in conjunction with the appended drawings in which: